1. Field of the Invention
The present invention is directed to the extrusion of materials such as molten thermoplastics, solutions such as viscose-like solutions which upon extrusion may be solidified by passage into a coagulation bath, cellulose acetate, paste-like slurries of materials which, upon extrusion, may become hardened and further solidified by heating or calcining where it is desired to impart a particular cross sectional shape to the form which is being extruded. This is accomplished by a two-part die design wherein one part of the die is machined along its edge to provide one half of the desired shape and the second part of the die which cooperates with the first die part is machined with the complementing part of the desired shape.
2. Background of the Invention
In a common technique employed for the extrusion of plastic and thermoplastic materials, the material to be extruded is fed to the hopper of a rotating screw type extruder. The rotating screw carries the material along to the terminal end of the extruder where it is expressed through a die orifice of some type. The material may be melted during its transit along the rotating screw by the heat generated as the material is compacted and subjected to the friction which exists between the extruder barrel on the screw or the material may be positively heated while it is within the extruder by heating jackets arranged externally on the surface of the extruder barrel. Conversely, in those cases where undesirably high levels of heat are generated in situ by the material being advanced between the screw and the extruder barrel, the external surface of the extruder barrel may alternatively be cooled to control the temperature of the material within the extruder.
In the extrusion of, for example, plastic films, it is known to cause the material as it leaves the surface of the extruder screw to pass into a die body and be expressed therefrom through a pair of closely aligned lips whereby a thin film is produced. Such extrusion of a flat film is referred to as either film extrusion or casting, the latter expression being taken from the fact that on extrusion from the slit orifice the semi-molten film is cast upon the surface of an internally cooled rotating roller to further cool the film and set it.
In the extrusion of synthetic textiles, it is known to extrude materials such as cellulose acetate, nylon, polyester, acrylics, modified acrylics and the like through a die which is referred to as a spinnerette. The spinnerette is a plate, usually a metallic plate, which is affixed to the end of the extruder through which the extruded material passes. Generally the plates are characterized by having a plurality of small circular orifices therein whereby cylindrical shaped fibers are extruded through the spinnerette plate. It has been known in the past that often times it is desirable to alter this cylindrical cross sectional configuration of the synthetic fibers being produced by altering the shape of the orifice in the spinnerette plate. Accordingly, a square fiber, a crescent shaped fiber, a multi-lobal fiber, a hollow fiber, and the like may all be produced by causing a design change in the individual orifices of the spinnerette. The reasons for the various designed cross sectional configurations in the synthetic fiber may be to form fibrous material which has an improved loft or handle or, for example, to improve the filtration characteristics of fibers which may eventually be employed in filtration operations.
Other materials which may be extruded through the spinnerette like plates described hereinabove include aqueous slurries of, for example, materials which will eventually be employed as catalysts in, for example, crude or refined lubricant processing. For example, it is known to form catalyst extrudate by forming a hydrated, powdered mixture of alumina, either by itself, or in admixture with other materials such as zeolites. This powdered mixture is fed to the hopper of a rotating screw-type extruder. The material charged is passed along by virtue of the rotating screw as it is being compacted and pressed and heated within the confines of the extruder barrel. This results in the powder being transformed into a plastic high-solids content material which is extrudable. The extrudable material is passed through an orifice in a die face plate (e.g., a spinnerette) and it is subsequently heated to relatively high temperatures, i.e., calcined to solidify and harden the materials.
It has been known in the past to produce such catalytic material with non-cylindrical cross-sectional characteristics by shaping the orifice in the die plate in accordance with the shape of the cross section desired for the final catalyst particle. Cross sectional shapes such as multi-lobal, cloverleaf and the like are desirable in certain catalyst applications, such shapes have proven themselves to be resistant to crushing under the high operating pressures that are sometimes encountered in refining operations. These shapes additionally offer a low pressure drop environment and high diffusion rates which in most instances are quite desirable in refining operations. Additionally, such variations in cross-section results in an increased surface to mass ratio which is advantageous in those applications when increased surface areas improve the catalytic process. U.S. Pat. No. 3,764,565 discloses various useful shapes of catalyst particles which may be formed utilizing the present apparatus.
In order to achieve the desired cross sectional configuration, an extrusion die with shaped holes is needed which will extrude the non-conventional, non-cylindrical forms of the catalyst. It has been found that conventional drilling machines have very limited use in order to achieve a complex shaped hole penetration through the steel die face necessary for the fabrication thereof. Other machining techniques such as electrical discharge machining or laser drilling must be used for holes which are non-circular. Such techniques are time consuming and expensive, and while the former technique is applicable to metals only, the latter is effective for relatively thin plates of non-metallic materials, for example.